For many years, carbon dioxide has been removed from gaseous mixtures with various absorbent liquids.
Alkali metal salts such as carbonates, phosphates, borates, and phenates of sodium and potassium are one category of absorbent liquid. The carbon dioxide absorption rates of such salts is, however, rather low, and, therefore, it has been necessary to add promoting agents to these salts. British Pat. No. 798,856 to S.P.A. Vetrocoke discloses that an inorganic or organic compound of trivalent arsenic is useful in activating such salts. In Astarita et al.'s "Promotion of CO.sub.2 Mass Transfer In Carbonate Solutions", Chemical Engineering Science, Vol. 36, pp. 581-88 (1981), it is mentioned that arsenious acid, ethanolamines, and amino acids promote the absorption of carbon dioxide by carbonate-bicarbonate salts. U.S. Pat. No. 4,094,957 to Sartori et al., U.S. Pat. No. 4,112,052 to Sartori et al., U.S. Pat. No. 4,217,237 to Sartori et al., U.S. Pat. No. 4,405,577 to Sartori et al., U.S. Pat. No. 4,405,578 to Sartori et al., U.S. Pat. No. 4,405,579 to Sartori et al., and Sartori et al.'s "Sterically Hindered Amines for CO.sub.2 Removal from Gases" , Industrial Engineering Chemical Fundamentals, Vol. 22, pp. 239-49 (1983) ("Sartori article") all disclose activating a basic salt for removing carbon dioxide from gaseous mixtures with sterically hindered amines or amino acids (i.e. a primary amine in which the amino group is attached to a tertiary carbon atom or a secondary amine in which the amino group is attached to a secondary or tertiary carbon atom).
Alkanolamines in aqueous solution are another class of absorbent liquid for removal of carbon dioxide from gaseous mixtures. Alkanolamines are classified as primary, secondary, or tertiary depending on the number of non-hydrogen substituents bonded to the nitrogen atom of the amino group. Monoethanolamine (HOCH2CH2NH2) is an example of a well-known primary alkanolamine. Conventionally used secondary alkanolamines include diethanolamine ((HOCH.sub.2 CH.sub.2).sub.2 NH) and diisopropanol amine ((CH.sub.3 CHOHCH.sub.3).sub.2 NH). Triethanolamine ((HOCH.sub.2 CH.sub.2).sub.3 N) and methyldiethanolamine ((HOCH.sub.2 CH.sub.2).sub.2 NCH.sub.3) are examples of tertiary alkanolamines which have been used to absorb carbon dioxide from industrial gas mixtures. These alkanolamines are not only useful in absorbing carbon dioxide, but they have also been employed to absorb hydrogen sulfide or carbonyl sulfide from gas mixtures which may or may not contain carbon dioxide.
After absorption of carbon dioxide and/or hydrogen sulfide and/or carbonyl sulfide in an alkanolamine solution, the solution is regenerated to remove absorbed gases. The regenerated alkanolamine solution can then be recycled for further absorption. Absorption and regeneration are usually carried out in different separatory columns containing packing or bubble plates for efficient operation. Regeneration is generally achieved in 2 stages. First, the absorbent solution's pressure is reduced so that absorbed carbon dioxide is vaporized from the solution in one or more flash regenerating columns. Next, the flashed absorbent is stripped with steam in a stripping regenerating column to remove residual absorbed carbon dioxide. With primary and secondary alkanolamines, the nitrogen reacts rapidly and directly with carbon dioxide to bring the carbon dioxide into solution according to the following reaction sequence: EQU 2RNH.sub.2 +CO.sub.2 .revreaction.RNHCOO.sup.- +RNH.sub.3
where R is an alkanol group. To obtain concentrations of carbon dioxide in solution which are greater than 0.5 mole of carbon dioxide per mole of alkanolamine, a portion of the carbamate reaction product (RNHCOO.sup.-) must be hydrolyzed to bicarbonate (HCO.sub.3.sup.-) according to the following reaction: EQU RNHCOO.sup.- +H.sub.2 O.revreaction.RNH.sub.2 +HCO.sub.3.sup.-
There is a characteristic equilibrium between the carbamate (RNHCOO.sup.-) and bicarbonate (HCO.sub.3.sup.-) ions for each alkanolamine which determines the vapor-liquid equilibrium or solution phase concentration of carbon dioxide for any given gas phase pressure of carbon dioxide. The alkanol substituent groups R which are attached to the nitrogen atom of any alkanolamine affect the basicity of the alkanolamine and its reactivity toward and vapor-liquid equilibrium with carbon dioxide.
In forming a carbamate, primary and secondary alkanolamines undergo a fast direct reaction with carbon dioxide which makes the rate of carbon dioxide absorption rapid. However, considerable heat is required to break the bond between the alkanolamine and carbon dioxide in the carbamate and regenerate the absorbent. In addition, primary and secondary alkanolamines have a limited capacity to absorb carbon dioxide due to the formation of stable carbamates. The Sartori article teaches that loading of such alkanolamines is improved by incorporating sterically hindered amines. Meanwhile, British Pat. No. 798,856 activates primary alkanolamines, like ethanolamine, with arsenious oxide.
Unlike primary and secondary alkanolamines, tertiary alkanolamines cannot react directly with carbon dioxide, because their amine reaction site is fully substituted with substituent groups. Instead, carbon dioxide is absorbed into solution by the following slow reaction with water to form bicarbonate: EQU R.sub.3 N+CO.sub.2 +H.sub.2 O.revreaction.HCO.sub.3.sup.- +R.sub.3 NH.sup.+
Because tertiary alkanolamines do not bond with carbon dioxide, they can be economically regenerated often by simply reducing pressure in the system (i.e. flash regenerating); little or no thermal regeneration is required. Although the absence of a direct reaction with carbon dioxide makes regeneration of tertiary alkanolamines more economical, large solvent circulation rates and high liquid to gas ratios (i.e. high liquid loadings) in the absorber are required due to the slow absorption of carbon dioxide. Consequently, systems utilizing tertiary alkanolamines require absorption columns of increased height and diameter compared to systems employing either primary or secondary alkanolamines.
In order to increase the rate of carbon dioxide absorption by aqueous tertiary alkanolamine solutions, promoters have been added. In U.S. Pat. No. 4,336,233 to Appl et al ("Appl patent"), a piperazine promoter is incorporated in an aqueous methyldiethanolamine solution. The process disclosed by the Appl patent "employs aqueous solutions of a bottom product obtained as a by-product from the synthesis of ethylenediamine from monoethanolamine and ammonia; this material also contains 0.3 percent by weight, based on piperazine of the following by-products: NH.sub.3, ethylenediamine, MEA, and further nitrogen-containing products." The by-products are merely said to "not interfere with the process according to" the Appl patent.
Promoted methyldiethanolamine solutions have both an increased rate of carbon dioxide absorption and an increased capacity for carbon dioxide compared to unpromoted methyldiethanolamine. Improved rate of absorption is particularly evident at low levels of carbon dioxide loading and diminishes as such loading increases. The full benefit of the promoter is found in processes which employ thermal regeneration in addition to flash regeneration of the absorbent to maintain the low loading levels necessary to produce a gas product with low levels of carbon dioxide. Flash regeneration alone is sufficient for bulk removal of carbon dioxide from high pressure gases (i.e. carbon dioxide partial pressure greater than 50 psia) where low carbon dioxide specifications in the product gas are not needed.